The attack was planned by a group of terrorists including Ramzi Yousef, Mahmud Abouhalima, Mohammad Salameh, Nidal A. Ayyad, Abdul Rahman Yasin and Ahmed Ajaj. They received financing from Khaled Sheikh Mohammed, Yousef's uncle. In March 1994, four men were convicted of carrying out the bombing: Abouhalima, Ajaj, Ayyad and Salameh. The charges included conspiracy, explosive destruction of property, and interstate transportation of explosives. In November 1997, two more were convicted: Ramzi Yousef, the mastermind behind the bombings, and Eyad Ismoil, who drove the truck carrying the bomb.

Yousef arrived illegally in the United States on September 1, 1992, traveling with Ahmed Ajaj from Pakistan, though both sat apart on the flight and acted as though they were traveling separately. Ajaj tried to enter with a forged Swedish passport, though it had been altered and thus raised suspicions among INS officials at John F. Kennedy International Airport. When officials put Ajaj through secondary inspection, they discovered bomb making instructions and other materials in his luggage, and arrested him. The name Abu Barra, an alias of Mohammed Jamal Khalifa, appeared in the manuals. Yousef tried to enter with a false Iraqi passport, claiming political asylum. Yousef was allowed into the United States, and was given a hearing date.[7]

Yousef set up residence in Jersey City, New Jersey, traveled around New York and New Jersey and called Sheikh Omar Abdel Rahman, a controversial blind Muslim cleric, via cell phone. After being introduced to his co-conspirators by Abdel Rahman at the latter's Al-Farooq Mosque in Brooklyn, Yousef began assembling the 1,500 lb (680 kg) urea nitrate–hydrogen gas enhanced device for delivery to the WTC. He ordered chemicals from his hospital room when injured in a car crash – one of three accidents caused by Salameh in late 1992 and early in 1993.

El Sayyid Nosair, one of the blind sheikh's men, was arrested in 1991 for the murder of RabbiMeir Kahane. According to prosecutors, "the Red" Mahmud Abouhalima, also convicted in the bombing, told Wadih el Hage to buy the .357 caliber revolver used by Nosair in the Kahane shooting. In the initial court case in NYS Criminal Court Nosair was acquitted of murder but convicted of gun charges (in a related and follow-up case in Federal Court, he was convicted). Dozens of Arabic bomb-making manuals and documents related to terrorist plots were found in Nosair's New Jersey apartment, with manuals from Army Special Warfare Center at Fort Bragg, North Carolina, secret memos linked to Joint Chiefs of Staff, and 1,440 rounds of ammunition. (Lance 2004 26)

According to the journalist Steve Coll, Yousef mailed letters to various New York newspapers just before the attack, in which he claimed he belonged to 'Liberation Army, Fifth Battalion'.[8] These letters made three demands: an end to all US aid to Israel, an end to US diplomatic relations with Israel, and a pledge by the United States to end interference "with any of the Middle East countries' interior affairs." He stated that the attack on the World Trade Center would be merely the first of such attacks if his demands were not met. In his letters Yousef admitted that the World Trade Center bombing was an act of terrorism, but this was justified because "the terrorism that Israel practices (which America supports) must be faced with a similar one."

On Friday, February 26, 1993, Ramzi Yousef and a Jordanian friend, Eyad Ismoil, drove a yellow Ryder van into Lower Manhattan, and pulled into the public parking garage beneath the World Trade Center around noon. They parked on the underground B-2 level. Yousef ignited the 20-foot fuse, and fled. Twelve minutes later, at 12:17:37 pm, the bomb exploded in the underground garage, generating an estimated pressure of 150,000 psi.[9] The bomb opened a 30-m (98 ft) wide hole through four sublevels of concrete. The detonation velocity of this bomb was about 15,000 ft/s (4.5 km/s). Initial news reports indicated a main transformer may have blown, before it became clear that a bomb had exploded in the basement.

The bomb instantly cut off the World Trade Center's main electrical power line, knocking out the emergency lighting system. The bomb caused smoke to rise up to the 93rd floor of both towers, including through the stairwells which were not pressurized, and smoke went up the damaged elevators in the World Trade Center Towers 1 & 2.[10] With thick smoke filling the stairwells, evacuation was difficult for building occupants and led to many smoke inhalation injuries. Hundreds were trapped in elevators in the towers when the power was cut, including a group of 17 kindergartners, on their way down from the South Tower observation deck, who were trapped between the 35th and 36th floors for five hours.[11][12]

Also as a result of the loss of power most of New York City's radio and television stations lost their over-the-air broadcast signal for almost a week, with television stations only being able to broadcast via cable and satellite via a microwave hookup between the stations and three of the New York area's largest cable companies, Cablevision, Comcast, and Time Warner Cable. Telephone service for much of Lower Manhattan was also disrupted.

All together, six people were killed and 1,042 others injured, most during the evacuation that followed the blast.[13] A report from the US Fire Administration states that "Among the scores of people who fled to the roofs of the towers, 28 with medical problems were airlifted by New York City police helicopters (...)". It is known that 15 people received traumatic injury from the blast and 20 complained of cardiac problems. One firefighter was hospitalized, while 87 others, 35 police officers, and an EMS worker were also injured in dealing with the fires and other aftermath.[14]

The plan was that if the bomb truck was parked at the right place, the North Tower would fall onto the South Tower, collapsing them both. However, the tower did not collapse, according to Yousef's plan, but the garage was severely damaged in the explosion. Nevertheless, had the van been parked closer to the WTC's poured concrete foundations, Yousef's plan might have succeeded.[15] He escaped to Pakistan several hours after the bombing.

Due to the time at which Yousef left Jersey City, questions linger as to why he waited until noon to attack. Author Simon Reeve theorized that something went wrong, such as Yousef encountering too much security, and the target was changed to be the World Trade Center.[13]

Yousef was assisted by Iraqi bomb maker Abdul Rahman Yasin, who helped assemble the complex 1,310-pound (590 kg) bomb, which was made of a urea nitrate main charge with aluminum, magnesium and ferric oxide particles surrounding the explosive. The charge used nitroglycerine, ammonium nitrate dynamite, smokeless powder and fuse as booster explosives.[16] Three tanks of bottled hydrogen were also placed in a circular configuration around the main charge, to enhance the fireball and afterburn of the solid metal particles.[17] The use of compressed gas cylinders in this type of attack closely resembles the 1983 Beirut barracks bombing 10 years earlier. Both of these attacks used compressed gas cylinders to create fuel-air and thermobaric bombs[18] that release more energy than conventional high explosives. According to testimony in the bomb trial, only once before the 1993 attack had the FBI recorded a bomb that used urea nitrate.[19][20]

The Ryder van used in the bombing had 295 cubic feet (8.4 m3) of space, which would hold up to 2,000 pounds (910 kg) of explosives. However, the van was not filled to capacity. Yousef used four 20 ft (6 m) long fuses, all covered in surgical tubing. Yasin calculated that the fuse would trigger the bomb in twelve minutes after he had used a cigarette lighter to light the fuse.

Yousef wanted the smoke to remain in the tower, therefore catching the public eye by smothering people inside, killing them slowly. He anticipated Tower One collapsing onto Tower Two after the blast.

There remains a popular belief that there was cyanide in the bomb, which is reinforced by Judge Duffy's statement at sentencing, "You had sodium cyanide around, and I’m sure it was in the bomb." However, the bomb's true composition was not able to be ascertained from the crime scene and Robert Blitzer, a senior FBI official who worked on the case, stated that there was "no forensic evidence indicating the presence of sodium cyanide at the bomb site." Furthermore, Yousef is said only to have considered adding cyanide to the bomb, and to have regretted not doing so in Peter Lance's book 1000 Years for Revenge.

Though the cause of the blast was not immediately known, with some suspecting a transformer explosion, agents and bomb technicians from the ATF, FBI, and the NYPD quickly responded to the scene. The magnitude of the explosion was far beyond that of a transformer explosion.[citation needed]

In the days after the bombing, investigators surveyed the damage and looked for clues. About 300 FBI agents were deployed under the codename TRADEBOM.[21] While combing through the rubble in the underground parking area, a bomb technician located some internal component fragments from the vehicle that delivered the bomb. A vehicle identification number (VIN), found on a piece from an axle, gave investigators crucial information that led them to a Ryder truck rental outlet in Jersey City. Investigators determined that the vehicle had been rented by Mohammad Salameh, one of Yousef's co-conspirators.[22] Salameh had reported the van stolen, and when he returned on March 4, 1993, to get his deposit back, authorities arrested him.[23]

The capture of Salameh and Yasin led authorities to Ramzi Yousef's apartment, where they found bomb-making materials and a business card from Mohammed Jamal Khalifa. Khalifa was arrested on December 14, 1994, and was deported to Jordan by the INS on May 5, 1995. He was acquitted by a Jordanian court and lived as a free man in Saudi Arabia until he was killed in 2007.[24] In 2002, it was made public that Yasin, the only person involved in the bombing who was never convicted by US authorities,[25] was being held as a prisoner on the outskirts of Baghdad, Iraq since 1994.[25] When journalist Leslie Stahl interviewed him there for a segment on 60 Minutes on May 23, 2002 [25] Yasin appeared in prison pajamas and handcuffs.[25] Yasin has not been seen or heard from since the interview. He was not located during the 2003 invasion of Iraq.

None of the U.S. government's indictments against former al-Qaeda leader Osama bin Laden suggested that he had any connection with this bombing.[26]

Stephen Knapp, age 47, Chief Maintenance Supervisor, Mechanical Section. Kirkpatrick, Macko and Knapp were eating lunch together in a room next to Smith's office at the time of the bombing.

John DiGiovanni, age 45, a dental products salesperson, was parking in the underground garage.

Wilfredo Mercado, age 37, a receiving agent for Windows on the World restaurant, was checking in deliveries.

A granite memorial fountain honoring the victims of the bombing was designed by Elyn Zimmerman and dedicated in 1995 on Austin J. Tobin Plaza, directly above the site of the explosion. It contained the names of the six adults who were killed in the attack as well as an inscription that read:

"On February 26, 1993, a bomb set by terrorists exploded below this site. This horrible act of violence killed innocent people, injured thousands, and made victims of us all."[27]

The fountain was destroyed with the rest of the World Trade Center during the September 11 attacks. A recovered fragment from the 1993 bombing memorial with the text "John D", from bombing victim John DiGiovanni, was later incorporated into a temporary memorial designed by Port Authority architect Jacqueline Hanley, and erected on the Liberty Street side of the site following the September 11 attacks. The memorial was visible across a fence barrier but was not open to the public.[28]

At the National 9/11 Memorial, which opened on the tenth anniversary of the 2001 attacks, the six adult victims of the 1993 bombing are memorialized at the North Pool, on Panel N-73.[29] The recovered fragment of the memorial fountain is on display inside the museum.

In the course of the trial it was revealed that the FBI had an informant, a former Egyptian army officer named Emad Salem. Salem claims to have informed the FBI of the plot to build a bomb that would eventually be used in the World Trade Center towers as early as February 6, 1992. Salem's role as informant allowed the FBI to quickly pinpoint the conspirators out of hundreds of possible suspects. The transcripts do not make clear the extent to which Federal Authorities knew that there was a plan to bomb the World Trade Center, merely that a bombing of some sort was being discussed.

Salem claimed that the FBI's plan was for Salem to supply the conspirators with a harmless powder instead of actual explosive to build their bomb, but that the FBI chose to use him for other purposes instead. He secretly recorded hundreds of hours of telephone conversations with his FBI handlers.[30]

Although the FBI received the credit, Diplomatic Security Service special agents actually found and arrested Ramzi Ahmed Yousef, the architect of the 1993 World Trade Center bombing. Special Agents Bill Miller and Jeff Riner were given a tip by an associate of Ramzi Yousef about his location. In coordination with the Pakistani Inter-Services Intelligence (ISI), DSS arrested Yousef.[31] After his arrest, Ramzi Yousef is alleged to have said to investigators "this is only the beginning."

In October 2001 in a PBS interview, former CIA Director James Woolsey claimed that Ramzi Yousef worked for Iraqi intelligence.[32] He suggested the grand jury investigation turned up evidence pointing to Iraq that the Justice Department "brushed aside." But Neil Herman, who headed the FBI investigation, noted "The one glaring connection that can't be overlooked is Yasin. We pursued that on every level, traced him to a relative and a location, and we made overtures to get him back." However, Herman says that Yasin's presence in Baghdad does not mean Iraq sponsored the attack: "We looked at that rather extensively. There were no ties to the Iraqi government." CNN terrorism reporter Peter L. Bergen writes, "In sum, by the mid-'90s, the Joint Terrorism Task Force in New York, the F.B.I., the U.S. Attorney's office in the Southern District of New York, the C.I.A., the N.S.C., and the State Department had all found no evidence implicating the Iraqi government in the first Trade Center attack."[33]

Claims of direct Iraqi involvement come from Dr. Laurie Mylroie of the American Enterprise Institute and former associate professor of the U.S. Naval War College, with the claims rejected by others. CNN reporter Peter Bergen has called her a "crackpot" who claimed that "Saddam was not only behind the '93 Trade Center attack, but also every anti-American terrorist incident of the past decade, from the bombings of U.S. embassies in Kenya and Tanzania to the leveling of the federal building in Oklahoma City bombing to September 11 itself."[33] Daniel Benjamin, a senior fellow at the Center for Strategic and International Studies, writes: "The most knowledgeable analysts and investigators at the CIA and at the FBI believe that their work conclusively disproves Mylroie's claims."[34] Dr. Robert Leiken of the Nixon Center comments on the lack of evidence in her work: "Laurie has discovered Saddam's hand in every major attack on US interests since the Persian Gulf War, including U.S. embassies in Kenya and Tanzania and even the federal building in Oklahoma City. These allegations have all been definitively refuted by the FBI, the National Transportation Safety Board (NTSB) and other investigatory bodies...."[35]

In March 2008, the Pentagon released its study of some 600,000 documents captured in Iraq after the 2003 invasion (see 2008 Pentagon Report). The study "found no 'smoking gun' (i.e., direct connection) between Saddam's Iraq and al Qaeda."[36] Among the documents released by the Pentagon was a captured audio file of Saddam Hussein speculating that the 1993 attack on the World Trade Center had been carried out by Israel or American intelligence, or perhaps a Saudi or Egyptian faction. Saddam said that he did not trust the bomber Yasin, who was in Iraqi custody, because his testimony was too "organized." The Pentagon study found that Yasin "was a prisoner, and not a guest, in Iraq."[37] Mylroie denied that this was proof of Saddam's non-involvement, claiming that "one common purpose of such meetings was to develop cover stories for whatever Iraq sought to conceal."[38]

In the wake of the bombing and the chaotic evacuation which followed, the World Trade Center and many of the firms inside of it revamped emergency procedures, particularly with regard to evacuation of the towers. Security ranged for the World Trade from WTC security guards to car barriers which would not allow cars to enter certain areas. The New York Port Authority was to govern as the main security for the World Trade buildings. All packages were scanned at various checkpoints then sent up to the proper addressee. These policies played a role in evacuating the building during the September 11th attacks, which destroyed the towers.

Free access to the roofs, which had enabled the evacuation by police helicopter in the 1993 bombing, was ended soon after.[citation needed]

The victims of the 1993 World Trade Center bombings sued the Port Authority of New York and New Jersey for damages. A decision was handed down in 2006, assigning liability for the bombings to the Port Authority. The decision declared that the agency was 68 percent responsible for the bombing, and the terrorists bore only 32 percent of the responsibility. In January 2008, the Port Authority asked a five-judge panel of the Appellate Division of the New York State Supreme Court in Manhattan to throw out the decision, describing the jury's verdict as "bizarre".[39] On April 29, 2008, a New York State Appeals Court unanimously upheld the jury's verdict. Under New York law once a defendant is more than 50 percent at fault, he/she/it can be held fully financially liable.[40] On September 22, 2011 - New York Court of Appeals, in a four to three ruling, excluded the Port Authority from claims of negligence related to the 1993 bombing.[41]

It has been argued that the problem with the apportionment of responsibility in the case is not the jury's verdict, but rather New York's tort-reform-produced state apportionment law. Traditionally, courts do not compare intentional and negligent fault. The Restatement Third of Torts: Apportionment of Liability recommends a rule to prevent juries from having to make comparisons like the terrorist-Port Authority comparison in this case. However, if a jurisdiction does compare these intentional and negligent torts, courts' second-best position is to do what the NYS Appeals Court did—to uphold all jury apportionments, even those that assign greater, or perhaps far greater, responsibility to negligent than intentional parties.[42]